When you enable synchronization on a router, this means that the IGP (iBGP in this case) and EGP must have the same information before those routes are used. The basic design
principal of a
transit AS is that you don’t advertise something until you can forward traffic to that
routefrom: http://www.cisco.com/en/US/tech/tk365/technologies_q_and_a_item09186a00800949e8.shtml#nineteen
If your AS passes traffic from another AS to a third AS, BGP should not advertise a route before all routers in your AS learn about the route via IGP. BGP waits until IGP propagates the route within the AS and then advertises it to external peers. A BGP router with synchronization enabled does not install iBGP learned routes into its routing table if it is not able to validate those routes in its IGP
and this excellent example...
http://ciscodreamer.blogspot.com/2009/03/bgp-synchronization.html
i'll have to see it for myself...
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